This invention relates to a power steering device, and more particularly to a power steering device wherein an input shaft and an output shaft are connected to each other through a torsion bar.
In a power steering device having a torsion bar incorporated therein, a resistance encountered during operation of steering wheels is partially caused due to a torsional resistance of a torsion bar. However, when the steering wheels are at a neutral position, they are free from the torsional resistance of the torsion bar, so that they are reduced in neutral rigidity, resulting in being deteriorated in stability during straight running or traveling of a vehicle.
In order to solve the problem, a power steering device which is constructed so as to apply preset force to the steering wheels to increase neutral rigidity thereof is proposed as disclosed in, for example, Japanese Patent Application Laid-Open Publications Nos. 507362/1994 and 507363/1994.
Now, the power steering device proposed will be described hereinafter with reference to FIG. 13. The power steering device proposed, as shown in FIG. 13, is so constructed that a support 101 associated with an output shaft (not shown) is arranged around an input shaft 102. The support 101 is formed with a plurality of slide holes or grooves 103, which are arranged in a manner to be spaced from each other at predetermined intervals in a circumferential direction of the support 101. The slide grooves 103 each have a plunger 104 slidably fitted therein. The plungers 104 each have elastic force applied thereto by means of C-shaped springs 105 arranged around the support 101. This results in each of the plungers 104 being forced toward a center of the input shaft 102.
The plungers 104 each are formed at a distal end thereof with a recess or cutout 106. Correspondingly, the input shaft 102 is formed on an outer peripheral surface thereof with cutouts or recesses 107, which are arranged so as to be opposite to the recesses 106 of the plungers 104 when the output shaft and input shaft are at a neutral position, respectively. Between each of the recesses 106 and the recess 107 corresponding thereto is arranged a roller 108.
In the power steering device thus constructed, elastic force of the C-shaped springs 105 is applied as the preset force to the input shaft 102 through the plungers 104 and rollers 108 during a neutral state of the steering wheels.
When the steering wheels are turned, followed by inputting of torque of a predetermined level thereto, the input shaft and output shaft are rotated relatively to each other while twisting the torsion bar 109. This causes each of the rollers 108 to be dislocated from the recesses 106 and 107, so that the rollers 108 each are moved between the outer periphery of the input shaft 102 and a rolling surface formed on a distal end of the plunger 104. Also, the conventional power steering device permits elastic force of each of the C-shaped springs 105 which acts as the preset force to act on the input shaft 102 through the plunger 104.
However, the plungers 104 each are required to be slidably fitted in each of the slide holes 103 formed in the support 101, resulting assembling of the power steering device being highly troublesome. Also, even when the slide hole 103 and plunger 104 are formed with increased dimensional accuracy, the plunger necessarily causes any backlash. Such backlash of the plunger 104 interferes with application of the preset force to the input shaft 102, so that the power steering device fails to exhibit its inherent or essential function. Also, it causes a variation in elastic force of the C-shaped springs 105 acting on the input shaft 102 during a rolling action of the rollers 108.